Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

A driving circuit (21) of a shape memory alloy actuator of the present
invention measures by a measurement part a parameter value corresponding
to a target position of a moving part that is displaced by being driven
on account of the expansion and contraction of an SMA (15), which expands
and contracts with temperature changes and which exhibits hysteresis in a
parameter-distortion characteristic relating to the expansion and
contraction, while the temperature of the SMA (15) is being raised or
lowered. The driving circuit (21) sets the measured parameter value as a
target parameter. The temperature of the SMA (15) is raised or lowered
such that the parameter value measured by the measurement part passes the
target parameter, before the crystal phase of the SMA (15) becomes a
martensitic phase. Thereafter, the temperature of the SMA (15) is raised
or lowered again such that the parameter value measured by the
measurement part reaches the target parameter.

Claims:

1. A shape memory alloy actuator drive device that drives a shape memory
alloy actuator having a shape memory alloy that expands and contracts on
account of heat generated through energization and that exhibits
hysteresis in a parameter-distortion characteristic relating to the
expansion and contraction, and a moving part that is displaced by being
driven on account of the expansion and contraction, the shape memory
alloy actuator drive device further having: a driving circuit that
performs the energization of the shape memory alloy; a measurement part
that measures a parameter relating to the expansion and contraction of
the shape memory alloy; a target displacement position detection part
that detects a target displacement position of the moving part; and a
control part that controls an value of energization current to the shape
memory alloy by the driving circuit in response to an output from the
measurement part and from the target displacement position detection
part, wherein the control part causes the moving part to be displaced in
one direction through sweeping of an increase and decrease of the
energization current value, in one direction, in the driving circuit when
during this time the target displacement position detection part detects
that the target displacement position has been passed, the control part
reads a measurement result of the measurement part, at that point in
time, as a target parameter, sets the target parameter to a value offset
by an overshoot amount that corresponds to an hysteresis amount of a
parameter-distortion characteristic that relates to the expansion and
contraction, upon causing the moving part to move in another direction by
changing the increase and decrease of the energization current value by
the driving circuit to be in another direction, and changes again the
increase and decrease of the energization current value to be in the one
direction, from a point in time at which the set value is obtained, in
order to cause thereby the moving part to move in the one direction and
be re-positioned to the target displacement position according to the
target parameter.

2. The shape memory alloy actuator drive device according to claim 1,
wherein the parameter relating to the expansion and contraction of the
shape memory alloy is temperature.

3. The shape memory alloy actuator drive device according to claim 1,
wherein the parameter relating to the expansion and contraction of the
shape memory alloy is a resistance value.

4. The shape memory alloy actuator drive device according to claim 1,
wherein the composition of the shape memory alloy is a Ni--Ti--Cu ternary
system including 3 at % or more of Cu.

5. An imaging device, using the shape memory alloy actuator drive device
according to claim 1.

6. A shape memory alloy actuator driving method for driving a shape
memory alloy actuator having a shape memory alloy that expands and
contracts on account of heat generated through energization and that
exhibits hysteresis in a parameter-distortion characteristic relating to
the expansion and contraction, and a moving part that is displaced by
being driven on account of the expansion and contraction, the shape
memory alloy actuator driving method comprising: a step of causing the
moving part to be displaced in one direction through sweeping of an
increase and decrease of the energization current value, in one
direction, in the driving circuit; a step of reading, upon detecting,
during the time at which the moving part is being displaced in the one
direction, that the moving part has passed the target displacement
position, a parameter value relating to the expansion and contraction of
the shape memory alloy at that point in time, as a target parameter; a
step of setting the target parameter to a value offset by an overshoot
amount that corresponds to an hysteresis amount of a parameter-distortion
characteristic that relates to the expansion and contraction, upon
causing the moving part to move in another direction by changing the
increase and decrease of the energization current value to be in another
direction; and a step of, while the moving part is being displaced in the
other direction, changing again the increase and decrease of the
energization current value to be in the one direction, from a point in
time at which the value set in the step is obtained, in order to cause
thereby the moving part to move in the one direction and be re-positioned
to the target displacement position according to the target parameter.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a drive device and method that are
appropriately used, for instance, in comparatively small imaging devices
provided in camera-equipped cell phones, for driving a shape memory alloy
actuator in order to adjust focus, zoom and so forth of a lens unit
forming an imaging optical system, and relates to an imaging device that
uses the drive device and method.

BACKGROUND ART

[0002] Image quality in imaging elements installed in camera-equipped cell
phones or the like has improved steadily in recent years as a result of,
for instance, dramatic increases in number of pixels. At the same time,
ever higher performance is demanded from lens units that are comprised in
imaging optical systems. Specifically, autofocus schemes are required
instead of fixed focal point schemes. As regards zoom performance,
optical zoom is now required as a substitute, or supplement, of digital
zoom. Both autofocus and optical zoom require an actuator that moves a
lens in the optical axis direction.

[0003] There are known devices that use shape memory alloys (hereafter,
also referred to as SMA) as such actuators. In such devices, a tightening
force is elicited in the SMA through ohmic heating. This tightening force
is used as a lens driving force that drives a lens. Usually, such a
configuration, which is also comparatively powerful, is advantageous on
account of the afforded miniaturization and weight reduction.

[0004] Ordinarily, however, shape memory alloys exhibit hysteresis in a
temperature--distortion characteristic, and the extent of distortion with
respect to temperature is dissimilar between a temperature-raising
process (heating process) and a temperature-lowering process (heat
release process). In the case of camera autofocus, the focus lens shifts
(heating) from a state in a home position (far end) in which impacts or
the like can be coped with (i.e. a state where the SMA is in a
martensitic phase (low-temperature phase)) and a state at a temporary
sweeping end (near end) (i.e. a state where the SMA is in an austenitic
phase (high-temperature phase)). A focus point is detected through, for
instance, detection of an edge at which contrast increases, on the basis
of the output of an image sensor during the time at which the focus lens
is shifting. After sweeping, the focus lens is positioned at the focus
point. If the SMA exhibits the above-mentioned hysteresis in a
temperature--distortion characteristic, therefore, the hysteresis curve
in a sweep (heating) from the far end to the near end is different from
the hysteresis curve in a return (heat release) from the near end to the
focus point). As a result, the focus lens stops in front of the target
position (i.e. does not return fully) when, during return, the control
part controls the energization of the SMA in such a manner that the SMA
temperature is brought to the temperature that is read during detection
of the focus point.

[0005] To cope with the above problem, control is carried out in Patent
document 1 so as to temporarily return the crystal phase in a shape
memory alloy to a martensitic phase (low-temperature phase), as
illustrated in FIG. 12. That is, control is performed such that heat
release takes place in the entirety of the shape memory alloy, regardless
of the target position (focus point). In other words, control is carried
out such that the focus lens returns temporarily up to a home position
(far end) at which the crystal phase is a martensitic phase
(low-temperature phase), and then the focus lens is caused to reach again
a target displacement (focus point) through a heating process of the
shape memory alloy.

[0006] In the above-described conventional technology, the focus lens is
caused to move to a target position (focus point) through a heating
process that is identical to the heating process at the time of sweeping
(focus point search). Therefore, it becomes possible to realize accurate
position control. However, substantial heat release time is required in
order to return to the martensitic phase. It takes also time to reach
again the target position (focus point). [0007] Patent document 1: WO
07/113,478

SUMMARY OF THE INVENTION

[0008] In order to solve the above issues, it is an object of the present
invention to provide a shape memory alloy actuator drive device and
method that allow realizing accurate position control in a shorter time,
and to provide an imaging device that uses the shape memory alloy
actuator drive device and method.

[0009] The shape memory alloy actuator drive device and method and imaging
device using the same according to the present invention are suitably
used in cases where a shape memory alloy expands and contracts with
changes in temperature and exhibits hysteresis in a parameter-distortion
characteristic relating to that expansion and contraction, in that: while
the temperature of the shape memory alloy is being raised or lowered,
there is measured a parameter value corresponding to a target position of
a moving part that is displaced by being driven on account of the
expansion and contraction of the shape memory alloy, and the measured
parameter value is set as a target parameter; the temperature of the
shape memory alloy is raised or lowered in such a manner that, before the
crystal phase of the shape memory alloy becomes a martensitic phase, the
parameter value measured by the measurement part passes the target
parameter; and thereafter, the temperature of the shape memory alloy is
raised or lowered again in such a manner that the parameter value
measured by the measurement part reaches the target parameter. In a shape
memory alloy actuator drive device having the above configuration,
therefore, the crystal phase of the shape memory alloy need not return to
a martensitic phase. Hence, accurate position control can be realized in
a shorter time.

[0010] The above and other objects and advantages will become more
apparent from the following description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a front-view diagram of an autofocus lens driving
mechanism of an imaging device according to an embodiment of the present
invention;

[0012] FIG. 2 is a side-view diagram for explaining the operation of the
autofocus lens driving mechanism illustrated in FIG. 1;

[0013]FIG. 3 is a graph illustrating a temperature--distortion
characteristic in a shape memory alloy, in an instance of displacement to
a target position through a heating process;

[0014]FIG. 4 is a graph illustrating a temperature--distortion
characteristic in a shape memory alloy, in an instance of displacement to
a target position through a heat release process;

[0015]FIG. 5 is a graph illustrating an operation example of an autofocus
sequence in an embodiment of the present invention;

[0016]FIG. 6 is a graph illustrating a displacement--driving current
characteristic and a displacement--temperature characteristic in a shape
memory alloy, depicting differences in displacement arising from
differences in ambient temperature;

[0017]FIG. 7 is a block diagram of a control circuit, in an embodiment of
the present invention, that drives an SMA actuator;

[0018]FIG. 8 is a flowchart for explaining autofocus control by the
control circuit illustrated in FIG. 7;

[0019]FIG. 9 is a block diagram of a control circuit, in another
embodiment of the present invention, that drives an SMA actuator;

[0020] FIG. 10 is a graph illustrating a displacement--resistance value
characteristic and a displacement--temperature characteristic of a shape
memory alloy;

[0021]FIG. 11 is a flowchart for explaining autofocus control by the
control circuit illustrated in FIG. 9; and

[0022]FIG. 12 is a graph illustrating an operation example of a
conventional autofocus sequence.

MODES FOR CARRYING OUT THE INVENTION

[0023] Embodiments of the present invention are explained below with
reference to accompanying drawings. In the drawings, identical features
are denoted with identical reference numerals, and an explanation thereof
will be omitted as appropriate.

Embodiment 1

[0024]FIG. 1 is a front-view diagram (diagram viewed from a lens aperture
plane) of an autofocus lens driving mechanism 1 of an imaging device
according to an embodiment of the present invention, and FIG. 2 is a
side-view diagram for explaining the operation of the autofocus lens
driving mechanism 1. FIG. 2(a) illustrates an instance where an SMA 15 is
stretched on account of the spring force of a bias spring 10, and FIG.
2(b) illustrates an instance where the SMA 15 is contracted against the
spring force of the bias spring 10. To perform focusing, the driving
mechanism 1 displaces a lens 2 in the direction of the axis line AX
(front-rear) of the lens 2. A lens barrel 4 comprises the lens 2 and a
lens driving frame 3, such that the lens 2 is attached to the lens
driving frame 3. A pair of protrusions 5 is formed at the front end
(front end in the front-rear direction) of the outer peripheral face of
the lens barrel 4. The protrusions 5 are set on an arm 12 of a shape
memory alloy actuator 11. Thereby, the lens barrel 4 is displaced in the
axis line AX (front/rear) direction.

[0025] The lens barrel 4 is placed on a base portion 6; the front and rear
ends of the lens driving frame 3 are supported by a base portion 6 and a
upper base 8, via a pair of link members 7, such that the lens barrel 4
can be displaced parallelly to the axis line AX (front-rear) direction.
The upper base portion 8 is integrally formed with the base portion 6 by
way of a horizontal-face outer wall, not shown. The bias spring 10 is
interposed between a front cover 9 and the front end of the lens driving
frame 3.

[0026] The shape memory alloy actuator 11 is provided with the arm 12, a
lever 13 and a support pedestal 14, as moving parts, and the SMA 15 that
comprises a wire of a shape memory alloy (SMA). The arm 12 is formed to a
substantially C-shape, as viewed from the front side (lens aperture), the
protrusions 5 are set on both sides of the arm 12, and the central
portion of the latter is fixed to one end of the lever 13. The central
portion of the lever 13 is supported on a fulcrum 14a of the support
pedestal 14 in such a manner that this lever can swing and shift. A
cutout 13a is formed at the other end of the lever 13. The SMA 15 is
wrapped around the cutout 13a. As a result, the cutout 13a prevents the
SMA 15 from being shifting upon displacement of the lens barrel 4 in the
axis line AX (front-rear) direction. Both ends of the SMA 15 are laid in
a tensioned state by a pair of electrodes 16 that are standingly provided
at the base portion 6.

[0027] In the above configuration, the SMA 15 releases heat naturally to
the surroundings, and is in a martensitic phase (low-temperature phase),
at a time where whether no current flows across the electrodes 16. Thus,
the SMA 15 generates no tension, and is stretched on account of the
spring force of the bias spring 10. Therefore, as illustrated in FIG.
2(a), the lens barrel 4 stands at a home position (far end) pressed
against the base portion 6, and can respond to a shock or the like. By
contrast, when current flows across the electrodes 16, for instance in a
pulsed manner, the SMA 15 generates Joule heat that is greater the higher
the duty ratio is. (The current flow amount is increased). The SMA 15
contracts on account of such self-heating, and tension is generated in
the SMA 15. The tension in the SMA 15 causes the lever 13 to swing in the
direction of arrow 18, as illustrated in FIG. 2(b), against the spring
force of the bias spring 10. As a result of this swinging, the lens
barrel 4 is pushed out towards the front cover 9, as indicated by arrow
19, by way of the arm 12 and the protrusions 5. In the state of highest
duty, the SMA 15 is in an austenitic phase (high-temperature phase), and
the lens barrel 4 reaches the sweeping end (near end).

[0028] In a lateral view (FIG. 2), the vicinities of inflection points of
the L-shaped lever 13 and of the arm 12 are supported on the fulcrum 14a.
The distance up to the point in the arm 12 at which the protrusions 5 are
locked is longer than the distance up to the point in the lever 13 at
which the SMA 15 is locked. As a result, the displacement of the SMA 15
is increased, and the abovementioned tension in the SMA 15 causes the
lens barrel 4 to be displaced.

[0029] FIGS. 3 and 4 are graphs illustrating the temperature--distortion
characteristic of an SMA (extent of displacement upon expansion and
contraction). When at or below a given temperature, the SMA is in a
crystal phase referred to as martensitic phase (low-temperature phase),
as indicated by the solid line, and the wire is stretched. As the
temperature rises, the SMA follows one branch of a hysteresis loop such
that, from a specific temperature (As point), the SMA contracts abruptly,
and displacement in the contraction direction increases. As the
temperature rises beyond a specific temperature (Af point), contraction
of the wire is over, and the SMA enters a crystal phase called an
austenitic phase (high-temperature phase). When temperature is lowered
from this state, the SMA follows the other branch of the hysteresis loop
and, from a specific temperature (Ms point), stretches abruptly, and
displacement in the contraction direction decreases. As the temperature
drops below a specific temperature (Mf point), stretching of the wire is
over, and the SMA returns to a martensitic phase. Ordinarily, the
temperature--distortion characteristic of SMAs exhibits hysteresis such
as the one illustrated in FIGS. 3 and 4, and there holds a relationship:
Mf point <As point and Ms point <Af point. On account of this
hysteresis, the SMA exhibits dissimilar displacement extents in a heating
process and a heat release process, for a same temperature.

[0030] In Patent document 1, accordingly, the SMA is controlled so as to
be displaced to a target position after the SMA has undergone complete
heat release and has returned temporarily to the martensitic phase
(low-temperature phase). In a case where the driving mechanism 1
illustrated in FIGS. 1 and 2 is used to perform such control, the
energization of the SMA 15 starts at time t0, as illustrated in FIG. 12,
and the lens barrel 4 starts moving at time t1. Once the lens barrel 4
starts moving (moving away from the home position), there is performed
focus evaluation over a plurality of steps, and the resulting evaluation
values (focus evaluation values) are stored. In this process, for
instance, there is detected a high contrast (edge), as a likely optimal
focus position, on the basis of a focus evaluation value acquired at time
t2. Thereafter, sweeping is performed over a predefined range, such that
sweeping is over at time t3. Determination of an accurate focus position
is performed before and after the above-mentioned time t2. Thereafter,
the SMA 15 is left to release heat over a sufficient lapse up time, up to
time t4, whereby the SMA 15 is caused to revert reliably to a martensitic
phase (low-temperature phase). Thereafter, energization starts again,
from time t4, so that by time t5, the lens barrel 4 has moved to an
accurate focus position, roughly that at the above-mentioned time t2.
Thereupon, the temperature at that time is maintained, and imaging is
performed.

[0031]FIG. 12 is a graph illustrating an operation example of a
conventional autofocus sequence. In FIG. 12, the left of the figure is a
SMA temperature--distortion characteristic, as in FIGS. 3 and 4 above.
Herein, the range from a manufacturer reference position (the
above-described home position) up to the near (macro) end is the
operation range, in order to define the motion range by the driving
mechanism 1. The right of the figure indicates the change in lens
position over the various processes of an autofocus sequence. The
abscissa axis is the time axis. The left and right of the figure have
been depicted in such a manner that the relationship between distortion
and displacement in the ordinate axes match each other. The same is true
of FIG. 5 of the present embodiment.

[0032] In the present embodiment, by contrast, energization of the SMA 15
starts at time t0, as illustrated in FIG. 5, the lens barrel 4 starts
moving at time t1, the optimal focus position is detected at time t2, and
sweeping is over at time t3. The heating process from time t0 to time t3
is identical to that of FIG. 12 described above. In the heat release
process that starts from time t3 in the present embodiment, a heating
process starts again, through an increase in the energization current
value of the SMA 15, from time t14 at which the optimal focus position
has been overshot by a predefined value Δ, and the autofocus
process is over upon reaching the optimal focus position at time t15,
whereupon imaging is carried out.

[0033] More specifically, returning to FIG. 3, the figure illustrates a
control operation wherein there is decided a given target position PT of
the heating process that constitutes one branch of a hysteresis loop;
thereafter, the lens barrel 4 is brought back through a heat release
process that constitutes the other branch of the hysteresis loop; is
returned to the target position PT, in a heated state, and is
re-positioned again. As indicated by the broken line, a heat release
process denoted by the reference numeral F1 takes place, to return from
the shift start position (sweep stop position) PM to the target position
PT. Herein, however, the SMA 15 is controlled to a temperature lower than
the temperature that corresponds to the target position PT of the heat
release process by the predefined value Δthat is based on the
above-described hysteresis characteristic of temperature--distortion that
is learned beforehand. Thus, after the heat release process has
temporarily overshot the target position PT, the SMA 15 is controlled
once more by way of a heating process, to reach a temperature
corresponding to the target position PT, as indicated by the reference
numeral F2. As a result, the driving mechanism 1 can accurately bring the
lens position to the target position.

[0034] Similarly, FIG. 4 illustrates an operation that involves deciding a
given target position PT in a heat release process, and performing
control thereafter so as to return to the target position, in a heat
released state. Although the heating and heat release relationship in
FIG. 4 is inverted with respect to that of FIG. 3, the driving mechanism
1 can accurately bring the lens position to the target position PT in
accordance with the same control method as that of FIG. 3.

[0035]FIG. 6 illustrates a displacement--driving current (ambient
temperature) characteristic and a displacement--temperature
characteristic of a SMA. The temperature of the SMA can be modified
through generation of Joule heat elicited by a driving current.
Therefore, the SMA exhibits fundamentally a hysteresis characteristic
identical to the displacement--temperature characteristic of FIGS. 3 and
4 above. However, the temperature of the SMA is affected by ambient
temperature, and hence the displacement--driving current characteristic
varies depending on the ambient temperature (depends on the ambient
temperature). When the ambient temperature is high, the driving current
is small for a same displacement, and when the ambient temperature is
low, the driving current is large for a same displacement. In order to
control the displacement of the SMA actuator 11, therefore, the driving
current must be appropriately controlled in accordance with such a
characteristic.

[0036] Thus, FIG. 7 illustrates a block diagram of a control circuit 21
that is a first drive device for driving the SMA actuator 11. The control
circuit 21 comprises a temperature sensor 22, a temperature detection
part 23, a microcomputer 24, an image sensor 25, a driving control
computation part 26 and a driving element 27. The control circuit 21
controls, by way of the driving element 27, the driving current that
flows in the SMA 15. In the control circuit 21, temperature is used as
the parameter that relates to expansion and contraction of the SMA 15,
i.e. as the parameter for detecting the position of the lens barrel 4.
Therefore, the temperature sensor 22 that constitutes a measurement part
is disposed in the vicinity of the SMA 15, such that the output of the
temperature sensor 22 is detected by the temperature detection part 23
and is inputted, in the form of a temperature detection value, to the
microcomputer 24. The temperature sensor 22 comprises, for instance, a
thermistor, a thermocouple, a thin-film resistor or the like, and is
provided in the lever 13 at a portion of the cutout 13a around which the
SMA 15 is wrapped.

[0037] The detection result of the image sensor 25 is inputted to the
microcomputer 24. On the basis of the output of the image sensor 25, the
microcomputer 24 determines the presence of a focus point upon detection
of an edge at which contrast becomes high. Therefore, the microcomputer
24 constitutes a control part as well as a detection part that detects
that the lens barrel 4 has reached a target displacement position. The
microcomputer 24 computes a driving current value in response to the
output of the image sensor 25 and the output of the temperature sensor
22, and supplies the results to the driving control computation part 26.
The driving control computation part 26 creates a duty driving signal
according to the driving current value, and controls the energization
current value (current value of the driving current) of the SMA 15 by way
of the driving element 27. Accordingly, a memory of the microcomputer 24
has stored therein a relationship between driving current, ambient
temperature and displacement, such as the one illustrated in FIG. 6
above, in such a way so as allow appropriately setting the energization
current value (duty) in accordance with a heating/heat release process
and the ambient temperature. In order to measure the ambient temperature,
there may be provided a separate temperature sensor, other than the
temperature sensor 22 that detects the temperature of the SMA 15.
Alternatively, the microcomputer 24 may be configured in such a manner
that the detection result of the temperature sensor 22 is read upon start
of energization of the SMA 15, and/or upon a pause of energization for a
predefined time, given that, ordinarily, ambient temperature does not
charge abruptly.

[0038]FIG. 8 is a flowchart for explaining autofocus control by the
control circuit 21. In FIG. 8 there is performed a focus position search
in a heating process such as the one illustrated in FIG. 3 above. Upon
start of the autofocus process, the microcomputer 24 firstly sets, in
step S1, the initial position at which the focus position search starts,
as an initial step position (position on the base portion 6 of FIG. 2).
In step S2, the microcomputer 24 acquires the ambient temperature by way
of the temperature sensor 22, via the temperature detection part 23. In
step S3, the microcomputer 24 decides a driving current value for causing
a lens to move to a next step position, on the basis of the acquired
temperature information and on the basis of a displacement--driving
current characteristic of the heating process that is stored beforehand.
The microcomputer 24 outputs the decided driving current value to the
driving control computation part 26.

[0039] In step S4, the driving control computation part 26 drives the SMA
15, via the driving element 27, at the abovementioned driving current
value. After a given time has elapsed, to allow for a required response
time for the motion of the lens barrel 4, the microcomputer 24 acquires,
in step S5, the temperature of the SMA 15 by the temperature sensor 22,
via the temperature detection part 23. In step S6, the microcomputer 24
performs focus evaluation on the basis of, for instance, contrast at that
step position, and stores the result together with the temperature of the
SMA 15 of step S5. In step S7, the microcomputer 24 determines whether or
not the present step position is an end position of the focus search. If
not, the process returns to the above-described step S3, in order to
change to a next step position. If, on the other hand, the microcomputer
24 determines that the step position is the end position, on the basis
of, for instance, the amount of defocus in the focus evaluation, the
microcomputer 24 terminates the heating process as the focus position
search, and, in step S8, sets the step position having a highest value,
from among the held focus evaluation values, as an optimal focus position
(target position) and sets the temperature at that step position to the
target temperature.

[0040] The various steps starting from step S8 above are processes of an
operation of moving to the target position. Firstly, the microcomputer 24
starts a heat release process from step S9. In step S9, the microcomputer
24 decides a driving current value, on the basis of the ambient
temperature obtained in step S2 and a displacement--driving current
characteristic of a heat release process, stored beforehand, in such a
manner that the target position is overshot by the predefined value
Δ, at a lower temperature than a temperature corresponding to the
target displacement. The microcomputer 24 drives then the SMA 15 in step
S10, in the same way as in step S4. In step S11, the microcomputer 24
measures the temperature of the SMA 15 in the same way as in step S5, and
in step S12, decides whether or not the temperature has reached the
temperature of the overshoot position set in step S9. If not, the process
returns to step S9 above. If yes, the heat release process is terminated,
and the process moves on to a re-heating process from step S13 onwards.

[0041] In step S13, in the same way as in step S3, the microcomputer 24
decides a driving current value corresponding to the target position, on
the basis of the ambient temperature obtained in step S2 and on the basis
of a displacement--driving current characteristic of the heating process
stored beforehand. The microcomputer 24 drives then the SMA 15 in step
S14, in the same way as in step S4. In step S15, the microcomputer 24
measures the temperature of the SMA 15 in the same way as in step S5, and
in step S16, decides whether or not the temperature has reached the
temperature of the overshoot position set in step S13. If not, the
process returns to step S13 above. If yes, the microcomputer 24 maintains
the temperature, terminates the re-heating process, and moves on to an
imaging operation.

[0042] In the flow of FIG. 8, the focus position search is performed in
the heating process illustrated in FIG. 3. In a case of focus position
search in the heat release process illustrated in FIG. 4, the autofocus
process can be carried out according to the same process, except that now
the above-described relationship of the heating/heat release process and
the temperature high/low relationship are reversed.

[0043] In the control circuit 21 of the present embodiment, thus, the
microcomputer 24 sweeps the value of energization current to the SMA 15
in one direction over a range defined beforehand, such that, upon
detection that the microcomputer 24 has passed a target position on
account of the resulting displacement of the moving part, the
microcomputer 24 reads the measurement result of the temperature sensor
22, at that point in time, as a target temperature corresponding to the
target position. To re-position the moving part to the target position
through a change in the energization current by the driving element 27 to
the other direction, in such a way so as reach the above-mentioned above
target temperature, the microcomputer 24 sets, as shown in FIG. 5, the
energization current value to a value offset by an overshoot amount
Δ corresponding to the amount of hysteresis in the
temperature--distortion characteristic of the SMA 15, and, from the point
in time at which the set value is obtained, the microcomputer 24 changes
again the energization current value to the above-mentioned one
direction, to reach the target position (temperature). Accordingly, the
control circuit 21 of the present embodiment allows realizing accurate
position control in a shorter time than in a method in which motion to
the target position is performed after the crystal phase has returned
temporarily to a martensitic phase (low-temperature phase), as in the
conventional case illustrated in FIG. 12.

[0044] A preferred composition of the SMA 15 is a Ni (nickel)-Ti
(titanium)-Cu (copper) ternary system comprising 3 at % or more of Cu.
That is because in a binary material of Ni--Ti alloy, the temperature
hysteresis is of about 20° C., but of about 10° C. in a
material of the above-mentioned Ni--Ti--Cu alloy ternary system. The
temperature hysteresis can thus be kept small. Time losses incurred on
account of the heating/heat release process can thus be cut by reducing
the above-described temperature hysteresis.

Embodiment 2

[0045]FIG. 9 is a block diagram illustrating the electric configuration
of a control circuit 31 being a second drive device for driving the SMA
actuator 11. The control circuit 31 comprises a resistance value
detection part 32, a comparator 33, a microcomputer 34, an image sensor
25, a driving control computation part 26 and a driving element 27. The
control circuit 31 controls the driving current that flows in the SMA 15
by way of the driving element 27. The control circuit 31 is similar to
the above-described control circuit 21, and hence corresponding features
will be denoted with the same reference numerals, and a recurrent
explanation thereof will be omitted. In the present embodiment 2, the
control circuit 31 uses a resistance value of the SMA 15 as a parameter
relating to expansion and contraction of the SMA 15, i.e. as a parameter
for detecting the position of the lens barrel 4. To that end, the
resistance value detection part 32 detects the resistance across the
electrodes 16 of the SMA 15, the comparator 33 compares the detection
result with a target resistance value given by the microcomputer 34, and
the driving current value is set in accordance with the compilation
result.

[0046] As described above, the driving current that flows from the
above-described driving element 27 to the SMA 15 is a constant current
the duty whereof changes according to the target position. As a result,
the resistance value detection part 32 can work out the resistance value
on the basis of a known constant current value during a period of ON
duty, and on the basis of the voltage across the electrodes 16 of the SMA
15. Alternatively, the resistance value detection part 32 causes a known
search current to flow in the SMA 15 during an OFF duty period at which
the driving element 27 is off and the driving current is not flowing. The
above-mentioned resistance value can then be worked out on the basis of
the voltage across the electrodes 16 of the SMA 15 that arises from the
search current.

[0047] FIG. 10 illustrates that displacement--resistance value
characteristic and a displacement--temperature characteristic of an SMA.
The resistance value of an SMA varies depending on the displacement, due
to the influence of changes in the crystal phase of the SMA and expansion
and contraction of the SMA. The displacement--resistance value
characteristic has a smaller hysteresis than a temperature--displacement
characteristic such as the one illustrated in FIG. 6 above, and is
fundamentally not affected by ambient temperature. Therefore, the
predefined value Δof the above-mentioned overshoot can be reduced
in a case where feedback control of the driving current is performed
using such a resistance value. Thus, positions can be detected in the
same way as described above by using a resistance value--distortion
characteristic of the SMA 15, without using any means for directly
learning the temperature. A relationship between a target resistance
value and displacement is stored beforehand in the microcomputer 34.

[0048]FIG. 11 is a flowchart for explaining autofocus control by the
control circuit 31. The operation in FIG. 11 is similar to that of FIG. 8
above, and hence identical operations will be denoted with the same step
number, while similar operations will be denoted with the same step
number with a prime ('). In the operation of FIG. 11 as well, a focus
position is searched in a heating process. Upon start of the autofocus
process, the microcomputer 34 firstly sets, in step S1, the initial
position at which the focus position search starts, as an initial step
position. In step S3', the microcomputer 34 decides a target resistance
value for causing a lens to move to that step position, on the basis of a
displacement--resistance value characteristic of the heating process
stored beforehand, and sets the decided target resistance value in the
comparator 33.

[0049] In step S4, the driving control computation part 26 drives the SMA
15, by way of the driving element 27, at a driving current value worked
out by the comparator 33 on the basis of the set target resistance value
and a present resistance value. After a given time has elapsed, to allow
for a required response time for the motion of the lens barrel 4, the
microcomputer 34 acquires, in step S5', the resistance value of the SMA
15 by the resistance value detection part 33. In step S6', the
microcomputer 34 performs focus evaluation on the basis of, for instance,
contrast at that step position, and stores the result together with the
resistance value of the SMA 15 of step S5'. In step S7, the microcomputer
34 determines whether the current step position is or not an end position
of the focus search. If not, the process returns to the above-described
step S3', in order to change to a next step position. If, on the other
hand, the microcomputer 34 determines that the step position is an end
position, on the basis of, for instance, the amount of defocus in the
focus evaluation, the microcomputer 34 terminates the heating process as
the focus position search, and, in step S8', sets the step position
having a highest value, from among the held focus evaluation values, as
an optimal focus position (target position), and sets the resistance
value at that step position as the target temperature.

[0050] The various steps starting from step S8' above are processes of an
operation of moving to the target position. Firstly, the microcomputer 34
starts a heat release process from step S9'. In step S9', the
microcomputer 34 decides a target resistance value being a resistance
value greater than a resistance value corresponding to a target
displacement, on the basis of the displacement--resistance value
characteristic of a heat release process, stored beforehand, in such a
manner that the position overshoots the target position by the predefined
value Δ. In step S10, the comparator 33 generates a driving current
value to drive thereby the SMA 15, in the same way as in step S4. In step
S11', the microcomputer 34 measures the resistance value of the SMA 15 in
the same way as in step S5', and in step S12', decides whether or not the
resistance value has reached the target resistance value of the overshoot
position set in step S9'. If not, the process returns to step S9' above.
If yes, the heat release process is terminated, and the process moves on
to a re-heating process from step S13' onwards.

[0051] In step S13', in the same way as in step S3', the microcomputer 34
decides a target resistance value corresponding to the target position,
on the basis of the displacement--resistance value characteristic of a
heating process stored beforehand. The microcomputer 34 drives then the
SMA 15 in step S14', in the same way as in step S4. In step S15', the
microcomputer 34 measures the resistance value of the SMA 15 in the same
way as in step S5', and in step S16', decides whether or not the
resistance value has reached the target resistance value of the target
position set in step S13'. If not, the process returns to step S13'
above. If yes, the microcomputer 34 maintains the resistance value,
terminates the re-heating process, and moves on to an imaging operation.

[0052] A configuration such as the above allows realizing accurate
position control, in a short time, on the basis of the resistance value
of the SMA 15.

[0053] The present description discloses various technical features as
described above. The main technical features involved are summarized as
follows.

[0054] A shape memory alloy actuator drive device according to one aspect
is a shape memory alloy actuator drive device that drives a shape memory
alloy actuator having a shape memory alloy that expands and contracts on
account of heat generated through energization and that exhibits
hysteresis in a parameter-distortion characteristic relating to the
expansion and contraction, and a moving part that is displaced by being
driven on account of the expansion and contraction, the shape memory
alloy actuator drive device further having: a driving circuit that
performs the energization of the shape memory alloy; a measurement part
that measures a parameter relating to the expansion and contraction of
the shape memory alloy; a target displacement position detection part
that detects a target displacement position of the moving part; and a
control part that controls an value of energization current to the shape
memory alloy by the driving circuit in response to an output from the
measurement part and from the target displacement position detection
part; wherein the control part causes the moving part to be displaced in
one direction through sweeping of an increase and decrease of the
energization current value, in one direction, in the driving circuit when
during this time the target displacement position detection part detects
that the target displacement position has been passed, the control part
reads a measurement result of the measurement part, at that point in
time, as a target parameter, sets the target parameter to a value offset
by an overshoot amount that corresponds to an hysteresis amount of a
parameter-distortion characteristic that relates to the expansion and
contraction, upon causing the moving part to move in another direction by
changing the increase and decrease of the energization current value by
the driving circuit to be in another direction, and changes again the
increase and decrease of the energization current value to be in the one
direction, from a point in time at which the set value is obtained, in
order to cause thereby the moving part to move in the one direction and
be re-positioned to the target displacement position according to the
target parameter.

[0055] In such a configuration, a shape memory alloy actuator comprises a
moving part that is displaced by being driven on account of expansion and
contraction of a shape memory alloy that expands and contracts on account
of heat generated through energization and that exhibits hysteresis in a
temperature--distortion characteristic and/or a resistance
value--distortion characteristic. Accordingly, the shape memory alloy
actuator drive device comprises a driving circuit that performs the
energization of the shape memory alloy; a measurement part that measures
a parameter of the shape memory alloy, for instance the temperature or a
resistance value, relating to the expansion and contraction; a target
displacement position detection part that detects a target displacement
position of the moving part; and a control part that controls an value of
energization current to the shape memory alloy by the driving circuit in
response to an output from the measurement part and from the target
displacement position detection part; the control part causes the moving
part to be displaced along one branch of a hysteresis loop, through
sweeping of the energization current value in one direction, over a range
established beforehand; when during this time the target displacement
position detection part detects that the target displacement position has
been passed, the control part reads a measurement result of the
measurement part, at that point in time, as a target parameter; and
changes the energization current value by the driving circuit to another
direction, in such a way so as to reach the target parameter, to move
thereby along the other branch of the hysteresis loop, and cause the
moving part to be re-positioned at the target displacement position. If
control is performed without taking the above hysteresis into account, an
offset in the actual displacement position arises between an instance
where one branch of the hysteresis loop is used and an instance where the
other branch is used. To achieve accurate positioning, therefore,
conventional control techniques resort to a method wherein shift to the
target position (the above-mentioned temperature or resistance value)
takes place after the crystal phase of the shape memory alloy has been
temporarily returned to a martensitic phase (low-temperature phase). Such
conventional techniques, however, require time for returning the crystal
phase to the martensitic phase. Therefore, the control part in the above
aspect causes the temperature to change up to substantially exceeding a
hysteresis amount of a parameter (the temperature or resistance
value)--distortion characteristic relating to expansion and contraction
of the shape memory alloy, and performs thereafter the shift to the
target position (temperature or resistance value).

[0056] As a result, the shape memory alloy actuator drive device having
such a configuration allows realizing accurate position control in a
shorter time.

[0057] In another aspect of the above-described shape memory alloy
actuator drive device, preferably, the parameter relating to the
expansion and contraction of the shape memory alloy is temperature.

[0058] Such a configuration allows controlling the expansion and
contraction of the shape memory alloy, and controlling the position of
the moving part of the shape memory alloy actuator, on the basis of
temperature.

[0059] In another aspect of the above-described shape memory alloy
actuator drive device, preferably, the parameter relating to the
expansion and contraction of the shape memory alloy is a resistance
value.

[0060] Such a configuration allows controlling the expansion and
contraction of the shape memory alloy, and controlling the position of
the moving part of the shape memory alloy actuator, on the basis of
temperature.

[0061] In another aspect of the above-described shape memory alloy
actuator drive devices, preferably, the composition of the shape memory
alloy is a Ni--Ti--Cu ternary system including 3 at % or more of Cu.

[0062] In such a configuration, the temperature hysteresis can be kept
small, since the temperature hysteresis of the Ni--Ti--Cu alloy is about
10° C. versus that of about 20° C. in a Ni--Ti alloy.

[0063] An imaging device according to another aspect uses any one of the
above-described shape memory alloy actuator drive devices.

[0064] In such a configuration, the imaging device can realize autofocus
quickly through the use of any one of the above-described shape memory
alloy actuator drive devices for focus lens driving control.

[0065] The present application is based on Japanese Patent Application No.
2008-327146, filed Dec. 24, 2008, the contents whereof have been
incorporated herein by reference.

[0066] The present invention has been appropriately and sufficiently
explained, by way of the above embodiments with reference to accompanying
drawings, so as to allow realizing the invention. However, it should be
noted that a person skilled in the art could easily conceive of
variations and/or improvements of the above-described embodiments.
Therefore, any variations or improvements that a person skilled in the
art could conceive of are meant to lie within the scope of the claims,
provided that such variations or improvements do not depart from the
scope of the claims.

INDUSTRIAL APPLICABILITY

[0067] The present invention succeeds in providing a drive device and
imaging device that utilize a shape memory alloy.